The present invention relates to an image forming apparatus, such as a copying apparatus, electronic printer, etc., and more particularly, to an image forming apparatus including a developing unit for developing an electrostatic latent image, by using a two-component developing agent formed of a toner and a carrier, and a toner supply unit for replenishing the developing unit with the toner.
In an image forming apparatus, e.g., a copying apparatus, comprising a developing unit for developing an electrostatic latent image by means of a two-component developing agent formed of a toner and a carrier, a toner supply unit is usually attached to the developing unit. The supply unit is used to replenish the developing unit with the toner.
Conventionally, however, the toner resupplied by the toner supply unit is of the same composition or type as the toner previously stored in the developing unit.
According to such a conventional method of toner supply, the image density increases as finished copies increase in number, as indicated by the line graph of FIG. 1. As indicated by the line graph of FIG. 2, moreover, the amount of electric charge on the toner diminishes in proportion to the increase of the number of finished copies.
In other words, as a copying cycle advances, accompanied by rotations of a developing roller and a stirring member and stress from a leveller (doctor blade), toner film may be formed on the carrier, or otherwise, highly-charged toner particulates may be fixed to the carrier. As a result, the carrier loses its primary effect of retaining electric charge of a predetermined level on the toner through its friction with the toner. Thus, it is impossible to obtain a proper amount of charge for the copying cycle.
Accordingly, the image density may be increased too much, or the inside of the apparatus may be soiled by bloom or scattered toner. Otherwise, toner particles of the opposite polarity, which cannot be used for the development, may increase, thus adding to toner consumption. Conventionally, therefore, the developing agent is lowered in life performance, and is replaced periodically. Alternatively, the developing agent is increased in quantity.
Referring now to FIGS. 3 and 4, the above description will be supplemented. All of the individual charged toner particles are not conducive to the development process. At least, toner particles of the opposite polarity cannot serve their purpose. In the vicinity of point 0 in FIG. 3, the toner particles have little, if any electric charge, so that they cannot adhere to the electrostatic latent image. This results in scattering of the toner particles. This tendency can be ascertained by comparing the graphs of FIGS. 3 and 4. As seen from these graphs, even if a toner of the same composition as the toner stored previously in the developing unit is used for replenishment, the amount of charge thereon varies gradually as the copying cycle advances. This is because the friction charging of the resupplied toner changes gradually.
Conventionally, as mentioned before, this problem is settled by using a short-life developing agent, which should be replaced periodically, or by increasing the quantity of the developing agent. However, such a counter-measure is not very economical, and entails an increase in apparatus size.